In recent years, flooding along Otterdale Road has occurred during periods of heavy rainfall leaving neighborhoods completely landlocked. This Design Build project consists of three different locations associated with the stream crossings of Blackman Creek, Horsepen Creek, and Otterdale Branch with Otterdale Road where bridge replacements are being proposed. The purpose of this project is to alleviate flooding and improve public safety at these three stream crossings. Development of the project in and of itself was unique, as the designer, JMT under contract with Chesterfield County, developed RFP plans for all three crossings however had to take the Horsepen Creek crossing design to 100% completion with an approved CLOMR prior to award to a Design Build team. Due to the nature of the site and flooding conditions, this project was an emergency type project of high priority to the County.
Review and analysis of historical and aerial mapping and photographs, as-built plans (only available for the existing bridge over Blackman Creek), existing FEMA studies, and other previous studies and plans developed by Chesterfield County and VDOT were part of the analysis. Blackman Creek and Horsepen Creek are in FEMA Flood Zones AE and Otterdale Branch is in a FEMA Flood Zone A. Site survey was supplemented with the best available current mapping and/or GIS data layers of the project site. The County requested that discharge values based on Zoning and Ultimate Development Land Use be calculated and used to design the bridges to ensure no possible overtopping of the road for the 100-year storm event.
There were several significant challenges on this project that required creative modeling approaches and techniques. The crossings showed significant sedimentation compared to the FEMA models, requiring context sensitive channel and overbank grading to improve hydraulic conveyance through the crossing, mitigate environmental impacts, remove the accumulation of existing debris and sediment, and to facilitate construction of the bridge abutments. Blackman Creek and Horsepen Creek also had reverse stream grades in existing and proposed conditions, which resulted in some innovative and effective modeling techniques. These challenges, amongst others such as stormwater management requirements and environmentally sensitive areas, were effectively overcome with creative and innovative solutions.